1. Field of the Invention
This invention relates to a phase change optical recording medium.
2. Prior Art
Highlight is recently focused on optical recording media capable of recording information at a high density and erasing the recorded information for overwriting. One typical rewritable (or erasable) optical recording medium is of the phase change type wherein a laser beam is directed to the recording layer to change its crystalline state whereupon a change of reflectance by the crystallographic change is detected for reproduction of the information. The phase change optical recording media are of great interest since the drive unit used for their operation is simple as compared with magneto-optical recording media.
Most optical recording media of phase change type used chalcogenide systems such as Ge--Sb--Te systems which provide a substantial difference in reflectance between crystalline and amorphous states and have a relatively stable amorphous state.
When information is recorded in the optical recording medium of phase change type, the laser beam applied is of high power (recording power) that the recording layer is heated to a temperature higher than the melting point. In the region where the recording power is applied, the recording layer is melted and thereafter quenched to form an amorphous record mark. When the record mark is erased, a laser beam of relatively low power (erasing power) is applied so that the recording layer is heated to a temperature higher than the crystallizing temperature and lower than the melting temperature. The record mark to which the laser beam of erasing power is applied is heated to a temperature higher than the crystallizing temperature and then allowed to slowly cool to recover the crystalline state. Accordingly, in the optical recording media of the phase change type, the medium can be overwritten by modulating the intensity of a single light beam.
In the optical recording medium of phase change type, dielectric layers are generally formed on opposite sides of the recording layer. Requirements for the dielectric layers are:
(1) the dielectric layers should be capable of protecting the recording layer and the substrate from heat histerisis as a result of the laser beam irradiation; PA1 (2) the dielectric layers should be capable of amplifying the reproduced signal by making use of optical interference effect of lights reflected from boundaries between the layers; and PA1 (3) the recording and erasing properties can be regulated by adjusting thermal conductivity and the like of each dielectric layer. PA1 condition I: light absorption coefficient of the recording layer at wavelength of the laser beam used for recording/reproduction is such that: EQU Ac/Aa.gtoreq.0.9 PA1 when light absorption coefficient in crystalline region is Ac and light absorption coefficient in noncrystalline region is Aa; and/or PA1 condition II: the medium has a light transmittance of at least 1% when the medium is irradiated from its rear side with a laser beam used for recording/reproduction; is fulfilled; and PA1 difference in reflectance between the record mark and the erased region after at least 10,000 overwriting operations is at least 85% of the difference in reflectance in initial overwriting operations. PA1 condition I: light absorption coefficient of the recording layer at wavelength of the laser beam used for recording/reproduction is such that: EQU Ac/Aa.gtoreq.0.9 PA1 when light absorption coefficient in crystalline region is Ac and light absorption coefficient in noncrystalline region is Aa; and/or PA1 condition II: the medium has a light transmittance of at least 1% when the medium is irradiated from its rear side with a laser beam used for recording/reproduction; is fulfilled; PA1 the first dielectric layer and the second dielectric layer contains zinc sulfide and silicon oxide as their main components, and the first dielectric layer comprises a dielectric layer 1a on the side of the substrate and a dielectric layer 1b on the side of the recording layer; PA1 the dielectric layer 1a has a silicon oxide content of from 2 mol % to less than 40 mol % (not including 40 mol %), the dielectric layer 1b has a silicon oxide content of from 40 mol % to 80 mol %, and the second dielectric layer has a silicon oxide content of from 40 mol % to 80 mol % when the silicon oxide content is determined by SiO.sub.2 /(ZnS+SiO.sub.2) by calculating the zinc sulfide and the silicon oxide in terms of ZnS and SiO.sub.2, respectively; and PA1 the second dielectric layer has a thickness of 10 to 35 nm.
Typical dielectric layers which meet such requirements are those containing highly refractive ZnS as their main component. For example, Japanese Patent Application Kokai (JP-A) No. 103453/1988 discloses an optical information recording material having a dielectric layer containing a mixture of ZnS and SiO.sub.2. The merits described therein include increase in sensitivity for the power of incident light upon recording, and increase in the number of erasing/overwriting operations of the dielectric material. The increase in the sensitivity is said to have been realized by optimizing thermal constant of the dielectric layer, and the increase in the number of erasing/overwriting operations is said to have been realized by preventing the alteration in the nature of the dielectric layer. JP-A 103453/1988 discloses that SiO.sub.2 /(ZnS+SiO.sub.2) is preferably in the range of 10 to 30 mol % since the laser energy required for the recording and the erasure is minimum when SiO.sub.2 /(ZnS+SiO.sub.2) is in such range.
However, in the optical recording medium of phase change type wherein the recording layer comprising Ge--Sb--Te based material or the like is sandwiched by dielectric layers containing Zns as their main component, C/N reduces with the repeated overwriting operations, and the medium becomes unoverwritable after approximately several thousand times. A major cause for the decrease in C/N with the repeated overwriting operations is believed to be influence of the alteration in the composition of the recording layer due to element diffusion between the adjacent dielectric layer.
JP-A 177141/1990 describes an optical information recording medium wherein reaction between the recording layer and the dielectric layers is suppressed. This optical information recording medium has a recording layer which absorbs light in the recording and erasure of the information, and a protective layer formed on at least one side of the recording layer, and the protective layer contains as its main components a metal chalcogenide and a compound which does not form a solid solution with the chalcogenide. This protective layer is formed such that the compositional ratio alters in the direction of the film thickness, and the part in the vicinity of the interface with the recording layer has a larger content of said compound. ZnS is the exemplary compounds disclosed as the chalcogenide and SiO.sub.2 is the exemplary compounds disclosed as said compound. JP-A 177141/1990 states that the reaction between the recording layer and the protective layer is prevented by increasing the SiO.sub.2 content of the protective layer in the vicinity of the recording layer and the alteration of the recording layer composition is thereby prevented to realize stable recording and erasure for about several million cycles with no substantial alteration in the reflectance. In Example 1 of JP-A 177141/1990, a first dielectric layer (thickness, 100 nm) containing ZnS and 20% of SiO.sub.2 is formed on the substrate, and a protective layer (thickness, 20 nm) containing at least 90% of SiO.sub.2, a recording layer of Te--Ge--Sb, the protective layer (thickness, 20 nm) as described above, a second dielectric layer (thickness, 200 nm) similar to the first dielectric layer, and a reflective layer of NiCr (thickness, 40 nm) are formed thereon. In Example 2 of JP-A 177141/1990, a protective layer having a composition gradient wherein SiO.sub.2 content is as high as 90% or more in the vicinity of the recording layer is formed instead of "the dielectric layer+the protective layer" of Example 1.
The optical recording medium of phase change type utilizes difference in reflectance between the crystalline and the noncrystalline state, and light absorption coefficient (Ac) of the recording layer in the region other than record marks (in crystalline state) and the light absorption coefficient (Aa) of the recording layer in the record marks (in noncrystalline state) are often different, and the Ac&lt;Aa is the condition generally found in such situation. Recording sensitivity and erasability are thus different depending on whether the region overwritten is crystalline or noncrystalline, and consequently, the record marks of different length and width are formed by the overwriting to invite increase in the jitter often resulting in errors. When mark edge recording wherein the information is encoded in opposite edges of the record marks is adopted for increasing the recording density, variation in the length of the record marks has greater significance and such variation invites increased errors. In order to solve such situation, the absorption coefficient should be adjusted such that the difference between Ac and Aa is small, and preferably Ac&gt;Aa in consideration of the latent heat, by regulating the thickness of the recording layer or the dielectric layers sandwiching the recording layer. In the medium of the conventional structure, the adjustment to Ac.gtoreq.Aa results in reduced difference between the reflectance (Rc) of the medium of the region other than the record marks and the reflectance (Ra) of the medium in the record marks, and hence, in a reduced C/N.
JP-A 124218/1996 proposes an optical information recording medium comprising a substrate, a first dielectric layer, a recording layer, a second dielectric layer, a reflective layer, a third dielectric layer, and a UV curing resin layer disposed in this order wherein Ac&gt;Aa, and an extremely thin metal film of high light transmission, Si or Ge is used for the reflective layer, and a dielectric material having a refractive index of higher than 1.5 is used for the third dielectric layer. The relation Ac&gt;Aa is achieved without detracting from the high (Rc-Ra) by providing the reflective layer of high light transmission and the third dielectric layer of high refractive index. In the present invention, the structure wherein difference between Ac and Aa is small or wherein Ac&gt;Aa is referred to as absorption coefficient control structure.
In the optical recording medium of absorption coefficient control structure, the dielectric layer is also preferably the one containing ZnS and SiO.sub.2 as its main components as in the case of the conventional optical recording medium. When the ZnS--SiO.sub.2 based dielectric layer is formed in the optical recording medium of absorption coefficient control structure, the medium also suffers from increase in jitter by the repeated overwriting operations as well as decrease in the modulation (difference between the reflectance of the crystalline portion and the reflectance in the non-crystalline portion), and as a consequence, decrease in the number of overwritable operations.
In view of such situation, the inventors of the present invention prepared an optical recording medium of absorption coefficient control structure in accordance with the Example of JP-A 177141/1990, supra. This optical recording medium, however, exhibited marked increase in jitter in the repeated overwriting operations, high jitter from the initial overwriting operations, and no improvement in the modulation decrease.